Repairing a Bowing Sheetmetal Roof

Technical Procedures Disclaimer

Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.


We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.

This procedure should only be performed by an experienced professional and only upon approval from the Regional Historic Preservation Officer (RHPO) or designated representative. This procedure should be performed under the direction of an historical architect or engineer to decide the most efficient and least destructive manner for executing the work.



A. This procedure includes guidance on refastening a bowing sheetmetal roof. Note: Generally, this work should be accomplished by an experienced roofing contractor.

B. Where cross welt clips have been omitted between pans of a sheet metal roof, the central zones of entire pans can be pulled upwards, eventually by as much as four to 6 inches. At this point upstands to standing seams and roll joints are pulled apart and the sheets of metal will be irreparably damaged. The amount of bowing varies depending on the metal used, and its weight.

C. Safety Precautions:

  1. Wear rubber-soled shoes that have nonslip or grid type tread (preferably sneakers with a high top for good ankle support). Avoid wearing loose clothing.
  2. Wear a safety belt or harness and secure it to a substantial chimney or to a window on the opposite side of the building. Leave only enough slack so you can work comfortably in one area, and adjust the slack as you work on other sections of the roof.
  3. Be sure the roof is clear of debris and water.
  4. Do not work on wet or snow covered roofs. Work on cleated walkboards.
  5. Steep roofs: On roof slopes greater than 4 inches rise per foot, special consideration must be given to both footing and materials handling.
    1. Secure chicken ladders or cleats at the top for adequate footing.
    2. Hang and secure approved safety lines with rope of sufficient strength.
    3. Carry a limited number of materials so that balance and footing are not impaired.
    4. Use scaffolding, ladders, and working platforms as required to execute the work. Ladders shall not be supported on hanging gutters. These gutters may be distorted which can affect the slope to drain.

D. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:

  1. Safety Precautions
  2. Historic Structures Precautions
  3. Submittals
  4. Quality Assurance
  5. Delivery, Storage and Handling
  6. Project/Site Conditions
  7. Sequencing and Scheduling
  8. General Protection (Surface and Surrounding)

These guidelines should be reviewed before performing this procedure and should be followed, when applicable, along with recommendations from the RHPO.


A. Anneal—the operation of heating and cooling the metal to soften it and make it less brittle.

B. Bay—a unit of sheet covering as laid between rolls or standing seams.

C. Cleats or clips—metal strips cut to lengths to suit roll or seam, placed at intervals and securely fixed to the roof base, the ends being welted in with the edges of the sheets to hold the sheetmetal roofing in position.

D. Flat seam—a seam between adjacent metal sheets, formed by turning up both edges, folding them over, and then flattening. A flat seam joint is usually soldered.

E. Standing seam—a seam between adjacent metal sheets, formed by turning up edges of two adjacent sheets, and then folding them over, but leaving them standing.

F. Batten seam—a seam in metal roofing that is formed around a wood strip.

G. Cross seam—cross seams are intended to provide the correct length for sheet metal pans, they should be staggered from bay to bay to make the seams stronger.

H. Pan—a formed metal sheet, usually about 21 inches wide by 28 inches long (maximum allowable length depends on the type of metal used). Includes both the flat sections and any upturns or folds required for the seams.

I. Welting—joining metal sheets at their edges by folding together. Welting may be single or double folds, called single and double welts respectively.

J. Dummy welt—folding of a long length of sheet metal without actually cutting the metal. The resulting appearance mimics a true welt.


A. Metal sheet pans should show no evidence of bowing or deformation in the central zone of the panel. All metal pans, clips or cleats, and all fasteners are of the same or compatible metal.


A. Storage and Protection

  1. Material storage: Keep uninstalled roof materials under cover, dry, free from scratches, condensation, and distortion during delivery, storage, and handling.
  2. Salvage storage: Historic material to be used as example of original construction shall be stored as directed by the RHPO. Often original roof metal scrap pieces with exposed weather can be found in attic spaces.
  3. Heavy bundles of nested panels require suitable mechanical equipment. Take care to prevent damage to corners and edges during handling or storage of metal roofing.
  4. Manufacturers’ delivery or job markings on metal, and adhesives for manufacturers’ labels shall be either a neutral or slightly acidic material. Never shall such material be alkaline; any staining of the metal by alkaline materials will be cause the rejection of the piece.
  5. Hoisting equipment and procedures will depend on the design of the panels, weight, and length.


A. Clean the roof of dirt build-up annually by rinsing with clean, clear water.

B. Keep the roof clear of debris, and trim all overhanging branches that might cause mechanical damage.

C. Inspect for and eliminate bird droppings and any other debris that can corrode sheet metals.

  1. Bird droppings can cause localized corrosion on sheetmetal because of the acids found in the droppings.
  2. Remove droppings using a wooden spatula; wash surface with a neutral detergent.
  3. Rinse with distilled water and wipe dry with a clean soft cloth, to prevent water spots and streaks.

CAUTION: Do not use bleach to remove bird excrement. Bird droppings contain ammonia and if mixed with bleach can form toxic gases.

D. Inspect the secureness of cleats and fasteners and the
condition of the sheet metal after particularly heavy storms.

E. Never use any black goop (asphaltic roofing compound) or caulk to seal joints on a metal roof. Asphalt attacks metal roofing, and no caulk lasts long enough for this application.



A. Nails and/or screws, metal to be compatible with roofing material

B. Cleats or clips, metal to be compatible with roofing material

C. Sheetmetal, to match type, weight/thickness, and size of original

D. Rosin Paper

E. Lumber for batten seams as appropriate


A. Chicken ladder, safety belt or harness.

B. Snips, as required, for cutting.

C. Hammer or screw driver.



A. Whenever possible, make inspection from ground, or from above if possible.

B. Indications of a bowed panel: Inspect for panels which do not lie flat, and which make a drumming noise in the wind. If the panel is bowed in the middle, the seams and fasteners will be under stress, and some may be undone or damaged.

C. Inspect the underside of the roof deck from the attic to detect leaks.


A. Protection: At the end of each work day, provide building protection for any exterior roofing element removed during repair.

B. Surface Preparation

  1. Carefully examine, measure, and record existing metal shingle or sheetmetal patterns at edges, hips, ridges, and other special conditions.
  2. Be careful not to damage old metal wall and vent flashing that may be used as a pattern for cutting templates. If metal cap flashing at the chimney and other vertical masonry wall intersections have not deteriorated, bend them up out of the way so that they may be used again. Carefully repair roofing in these areas to avoid damaging reusable base flashing.
  3. Inspect the deck to determine whether it is sound. Make whatever repairs are necessary to the existing roof framing to strengthen it and to level and true the deck. Replace rotted, damaged, or warped sheathing or delaminated plywood material.
  4. For installation of new material, verify the type, thickness, weight/gauge before installation.
  5. Before installation, remove all oil, dirt, and other debris from the surface. All surfaces shall be dry and free from frost.
  6. For safety of the personnel, keep the roof clear of waste material as the work proceeds.
  7. Only work on a quantity of roofing which may be repaired on that same day.


A. If the damage is discovered early, when deformation is no more than 1-1/4 inches, several options are available.

  1. For small areas, 2–3 pans:
    1. Carefully pry open the cross welts.
    2. Anneal each opened joint. Use damp rags to keep the surrounding metal cool where it is in contact or close to the wood roof decking.
    3. Insert a 2" wide clip into the crosswelt. Make sure the metal used is compatible with the metal of the roof.
    4. Fasten the clip to the understructure with nails or screws, again they should be compatible with the metal of the roof.
    5. Reanneal all edges and close all seams as originally finished.
  2. For a large area: Each bowed area or bay is cut down the center, its entire length, from ridge to eaves. The edges are turned up to form upstands as required by either a standing seam or batten seam, depending on the seam type of the remainder of the roof.
    1. a. Standing seam method:
      1. Prepare a new narrow strip of appropriate sheetmetal with upstands to complement those of existing sheets. Lay over new rosin paper or other appropriate underlayment.
      2. Space new clips at 15 inches on center along the length of the new seams.
      3. Fasten clips to deck with two nails or two screws per clip, using fasteners compatible with the clip metal. Clips should not be placed at junctions with cross welts to avoid building up an unmanageable thickness of metal in the standing seams.
      4. Provide for longitudinal expansion by using dummy welts.
    2. Batten seam method:
      1. Fasten a new timber batten into the decking with countersunk head steel screws (screw must not contact capping material).
      2. Insert clips under the batten, turn up on each side, and fold in with the new capping strip welt. Make sure metal of clip is the same metal as the roof.
      3. New capping strips to cover the top of each batten are to be formed from a fully annealed metal strip the same thickness as the existing roof covering and the same metal.